This invention relates to the purification of aqueous hydrogen sulfide-scrubbing or washing solutions and is particularly concerned with processes for treating such solutions so that they can be safely and legally discharged into the environment.
One area of particular concern in the oil and chemical industries is the release of sulfur and its compounds into the atmosphere as the result of the refining of petroleum, the sweetening of sour and natural gas, the processing of ores, the destructive distillation of coal and/or oil shale, the gasification or liquefaction of coal, the use of geothermal fluids to generate electricity or other processes which produce hydrogen sulfide-containing gases.
Various processes have been employed to remove hydrogen sulfide from gas streams. These processes include, among others, the Stretford process, the Takahax process and the Unisulf process. All three of these processes employ aqueous alkaline scrubbing or washing solutions to absorb the hydrogen sulfide from the gas stream and to convert the absorbed hydrogen sulfide primarily to elemental sulfur. The spent solution is regenerated by flowing air through the solution. The product sulfur is separated from the regenerated solution before recycling the solution to the absorber. The scrubbing solution employed in the Stretford process contains a water-soluble salt of anthraquinone disulfonic acid (ADA) and a water-soluble vanadate. The washing solution employed in the Takahax process contains a water-soluble naphthoquinone sulfonate alone or in combination with a multivalent metal compound. The Unisulf scrubbing solution, like the Stretford scrubbing solution, contains a water-soluble vanadate and a water-soluble salt of ADA or other aromatic sulfonate, such as a water-soluble salt of phenolsulfonic acid. In addition, the Unisulf solution also contains thiocyanate ions and a carboxylate complexing agent such as citric acid.
While the above-discussed processes and other hydrogen sulfide-scrubbing processes have been successful in substantially reducing the hydrogen sulfide content of the gas streams being treated, there are problems associated with the processes. One common problem involves the undesirable conversion of the absorbed hydrogen sulfide and/or the product sulfur to water-soluble sulfur-containing salts such as sulfates and thiosulfates which accumulate in the washing solution. These accumulated salts tend to (1) increase the corrosivity of the scrubbing solution, (2) cause precipitation of the desired constituents of the scrubbing solution and (3) increase the rate of chemical consumption of the alkaline constituents of the scrubbing solution. Because of this, Stretford solutions must be replaced about every six to eighteen months while Unisulf solutions may need replacement every five or more years. Traditionally, the spent scrubbing solutions are disposed of by sending them to toxic waste landfills. Disposal in this manner is not only expensive, costing between one and two million dollars per year for the disposal of spent Stretford solutions by a typical refinery, but also raises certain risks.
These risks include the fact that the Environmental Protection Agency (EPA) has started to force the cleanup of such landfills under the Comprehensive Environmental Response Compensation Liability Act (CERCLA or Superfund) and the Superfund Amendment and Reauthorization Act of 1986 (SARA). In addition, the Hazardous and Solid Waste Amendments (HSWA) to the Resource Conservation and Recovery Act (RCRA) forbid disposal of certain substances in toxic waste landfills. This list of substances is likely to grow as regulations become stricter. Finally, the EPA is especially strict on disposal of liquids in toxic waste landfills because of the high probability that a liquid will enter the water table. In view of the legislative activity in this area, there is a high probability that the disposal of aqueous hydrogen sulfide-scrubbing solutions in toxic waste landfills may not be legal in the future.
Because of the costs and risks of disposing of spent, aqueous, hydrogen sulfide-scrubbing solutions in toxic waste landfills, there is a need for alternative means of disposal. Accordingly, it is a primary object of the present invention to provide a process for treating such solutions to purify them so that the resultant wastewater can be safely discharged into the environment. It is another object of the invention to provide such a process that can be used in conjunction with existing commercial processes so that the cost of disposal is substantially reduced. These and other objects of the invention will become more apparent in view of the following description of the invention.